Rotor blades on military helicopters are routinely removed and installed during maintenance, storage and transportation. Dismantling of a main rotor blade involves the attachment of a blade clamp to the lateral center of gravity of the blade, lifting the blade clamp with a hoist to facilitate removal of the blade from the rotor and then hoisting it away from the helicopter for storage.
The cross section of a typical rotor blade is a classic airfoil design with a rounded leading edge and a sharp trailing edge. The new Blackhawk M model helicopter implements a wide-chord composite main rotor blade containing a trim tab along its trailing edge. “Wide-chord” is a term used to refer to the width of the blades. Composite blade designs are fragile and require sensitive clamping so that the blade is not damaged. The delicate trim tabs on the trailing edge are made with precision, perhaps from a costly material such as titanium alloy strip, for example. By virtue of the blade design, the trim tab is placed at the center of gravity of the blade, which coincidentally is also the blade clamping location.
Securing a rotor blade with a clamp at its center of gravity and connecting a hoist or sling to the clamp to support its weight is known. Blade clamps provide a way to secure or move helicopter blades in order to protect the blades during storage/transit of the helicopter. Protection of a helicopter blade during its removal or installation is critical since many present day helicopter blades are designed with lighter but more fragile materials, allowing for the greater possibility of costly damage. Likewise, modern helicopter blades also require carefully engineered clamps to ensure that damage to the blade does not occur during removal and installation of the blade.
A typical blade clamp is expected to secure the blade at its lateral center of gravity, and when lifted, the blade must hang level from a hoist. Transportation and handling of the blade using the blade clamp must be carried out without any risk of damage to the blade and its accessories. During the clamping operation, the blade clamp must not damage the blade, and thus the blade clamp must avoid certain structurally-sensitive areas on the blade during clamping and removal. Moreover, the clamping pressure must not damage the structure of the blade. The clamping mechanism must be secure, and the structural components used for the blade clamp must have adequate material properties and room temperature durability for the life of the clamp.
Known attempts at a helicopter blade clamp such as U.S. Pat. No. 4,301,982 to Tiemann, feature a leading edge design where the clamp hinge is situated on the leading edge of the blade. Leading edge designs, while able to avoid contact of sensitive trim tabs located on the trailing edge of some types of blades, prevent the blades from entering the blade cradle in a skid.
Additionally, known clamps such as Tiemann, clamp only upon part of the blade, and not along the entire width. This prevents any clamp mechanism from contacting and damaging the sensitive trim tabs on the trailing edge, but at the expense of not evenly distributing the restraining force along the width of the blade. Moreover, because the known blade clamps extend only half-way across the cross section in a clamping-ready position from the leading edge, the blade is not fully secured, resulting in blades being dropped during handling. To prevent this problem, a safety strap was later added to the known blade clamps; however the strap could not be used on blades with trim tabs located at the lateral CG.
Ease of use of known blade clamps is also a problem. Known clamps often require at least three people to maneuver the unclamping of blades and their placement in skids. Moreover, the clamps are too heavy for field use and none of the current clamp designs interface with the blade storage cradle. With the known clamps, removing the blade typically takes 10-15 minutes. First the blade clamp is secured and then a safety strap has to be tied around the trailing edge of the blade. Clamping the blade is done by feel and experience, and any slight mistakes in the clamping process can damage the blade. Once the blades are removed, they are placed with the leading edge down in narrow slots or cradles in a skid. Placing the blade in the skids is challenging, in that the clamp has to be removed at a pre-designated location way from the skid. Then the blade has to be physically lifted by two individuals and placed in the cradle.
Moreover, many known clamps are suited to fit only one type of blade width and cannot safely be used interchangeably with other models of helicopter blades.
Known blade clamps suited for the A/L model Blackhawk helicopter blades have a narrow width. Out of necessity, efforts have been made to use the legacy blade clamps on the newer wide-chord blades. However, special blade clamping challenges present with the new Blackhawk M model blade design prevent these known clamps from being suitable for use.
The older, existing A/L blade clamp requires use of a safety strap due to occurrences of clamp failure. Trim tab location of the M model blades falls at the blade's center of gravity, the necessary location for clamping. The straps on the existing blade clamp damage the trailing edge. The clamping force used to secure the clamp on the blade creates a line force on the surface of the blade that may damage the blade and its sub-surface structure.
What is needed is an ideal new blade clamp that would be lightweight and easily maneuverable such that it could be lowered and unclamped directly into the cradle by a single individual. Also the ideal blade clamp would be easily maneuverable when removing the blade from the cradle for installation on a helicopter. The ideal blade clamp would be a design with features that make it universally adaptable across other platforms with minor modifications. The ideal blade clamp would be capable of clamping with even force across the width of the blade, clamping on the center of gravity located at the trim tabs, but without damaging the trim tabs. The ideal blade clamp would be capable of all the above features while still meeting certain benchmarked structural requirements.